In automation technology, especially in process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are sensors integrated, for example, in fill level measurement devices, flow measuring devices, pressure- and temperature measuring devices, pH, redox-potential measuring devices, conductivity, measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, redox-potential, and conductivity, respectively. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, respectively the fill level in a container, can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and deliver, or process, process relevant information. In connection with the invention, the terminology, field devices, thus includes also remote I/Os, radio adapters, respectively, in general, electronic components, which are arranged at the field plane. A large number of such field devices are produced and sold by the firm, Endress+Hauser.
Depending on application, field devices must work under the most varied of environmental conditions. A basic assumption is, however, that the devices are resistant against external electromagnetic disturbing influences. This is referred to as electromagnetic compatibility, or EMC for short. The required electromagnetic compatibility of electrical measuring-, testing- and control devices is specified, for example, in the standard, DIN EN 61326. This relates, among other things, to the high-frequency region between 100 MHz and 10 GHz. An essential contribution to the electromagnetic compatibility of field devices in this frequency range is provided by the housing. To the extent that the housing is designed as a Faraday cage, the housing interior is protected against EMC disturbance signals in this frequency band. Necessary for this is that at least the housing interior or exterior surface is as conductive as possible.
If the housing is composed of a number of subcomponents, then these subcomponents must supplementally be electrically connected with one another with one another by low impedance junctions for the referenced frequency range. This holds especially for the housing lid, through which the device interior is accessible when necessary.
In the case of many housing types, the connection between housing lid and housing is implemented as a screwed connection. In choosing the screw thread type, in such case, usually the achieved force interlocking between housing and housing lid is the decisive criterion, in order, for example, to achieve a hermetic closure. In the case of metal housings, consequently, preferably triangular screw threads are used, while, in the case of plastic housings, for the purpose of sufficient stability for the force interlocking, as a rule, trapezoidal threads according to DIN 6063-2 are used.
Disadvantageous in the case of these screw thread types is, however, the insufficient transition-impedance for high-frequency electromagnetic disturbance signals between external and internal thread in the mentioned frequency range. This insufficient transition-impedance results from the skin-effect. This effect describes that high-frequency signals are conducted in an electrical conductor only near to the surface. In the case of blocky screw thread geometries such as triangular or trapezoidal threads, this leads to a high transition impedance and, thus, to poor draining of the EMC disturbance signal, so that the EMC protection function of housing is only insufficiently executed.
In contrast therewith, screw thread types with rounded thread side-form have significantly smaller transition impedances. From this point of view, round profiles under DIN 405 provide very advantageous impedance values. However, the force interlocking in the case of this screw thread type is, as a rule, extremely small.